The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons D. McComas1,*, F. Allegrini1, F. Bagenal2, P. Casey1, P. Delamere2, D. Demkee1, G. Dunn1, H. Elliott1, J. Hanley1, K. Johnson1, J. Langle3,1, G. Miller1, S. Pope1, M. Reno1, B. Rodriguez1, N. Schwadron4,1, P. Valek1, S. Weidner1 1Southwest Research Institute®, San Antonio, Texas 78228-0510, U.S.A. 2University of Colorado, Boulder, Colorado 80309, U.S.A. 3Midwest Research Institute, Kansas City Missouri 64110, U.S.A. 4Boston University, Boston Massachusetts 02215, U.S.A. ( *author for correspondence, e-mail: [email protected]) Abstract. The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. In order to meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer (RPA) with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft. Keywords: solar wind, Pluto plasma interaction, pickup ions 1.0 Introduction The New Horizons mission [Stern et al., 2007] will make the first up-close and detailed observations of Pluto and its moons. These observations include measurements of the solar wind interaction with Pluto. The Solar Wind Around Pluto (SWAP) instrument is designed to measure the tenuous solar wind out at ~30 AU and its interaction with Pluto. SWAP directly addresses the Group 1 science objective for the New Horizons mission to “characterize the neutral atmosphere of Pluto and its escape rate.” In addition, SWAP makes measurements critical for the Group 2 objective of characterizing Pluto's 1 ionosphere and solar wind interaction. Finally, SWAP makes observations relevant to two Group 3 science objectives for New Horizons, both in support of characterizing the energetic particle environment of Pluto and Charon and in searching for magnetic fields of Pluto and Charon. As the solar wind approaches Pluto, it interacts with ions produced when Pluto’s thin upper atmosphere escapes, streams away as neutral particles, and becomes ionized. This pickup process slows the solar wind, and the type of interaction varies greatly depending on the atmospheric escape rate. SWAP measurements should provide the best estimate of the overall atmospheric escape rate at Pluto and allow the first detailed examination of its plasma interactions with the solar wind. The SWAP observations are extremely challenging because the solar wind flux, which falls off roughly as the square with distance, is approximately three orders of magnitude lower at Pluto compared to typical solar wind fluxes observed near Earth’s orbit. In addition, because the solar wind continues to cool as it propagates out through the heliosphere, the solar wind beam becomes narrow in both angle and energy. The SWAP design was strongly driven by three constraints: 1) very low use of spacecraft resources (mass, power, telemetry, etc.) as this is a non-core instrument on a relatively small planetary mission; 2) very high sensitivity to measure the solar wind and its interaction with Pluto out at ~30 AU, where the density is down by a factor of ~1000 compared to 1 AU; and 3) the need to make observations over a very large range of angles as the spacecraft constantly repoints its body- mounted cameras during the flyby. Given these not-entirely-consistent design drivers, we developed an entirely new design that combines elements of several different previous plasma instruments. Together these components comprise the SWAP instrument, which will measure the speed, density, and temperature of the distant solar wind and its interaction with Pluto. 2 2.0 Scientific Background and Objectives Only partly in jest, Dessler and Russell (1980) suggested that Pluto might act like a colossal comet. The 1988 stellar occultation showed that Pluto’s tenuous atmosphere could indeed be escaping (Hubbard et al., 1988; Elliot et al., 1989). Applying basic cometary theories to Pluto, Bagenal and McNutt (1989) showed that photoionization of escaping neutral molecules from Pluto’s atmosphere could significantly alter the solar wind flow around Pluto for sufficiently large escape rates. For large atmospheric escape rates, the interaction may be best described as “comet-like,” with significant mass-loading over an extensive region; for small escape rates the interaction is probably confined to a much smaller region, creating a more “Venus-like” interaction (Luhmann et al., 1991), where electrical currents in the gravitationally bound ionosphere deflect the solar wind flow. Figure 1 compares these two types of interactions schematically. At aphelion (50 AU), should Pluto’s atmosphere completely collapse and freeze onto the surface, then the interaction becomes “Moon-like” with the solar wind suffering minimal deflection and directly bombarding the bare, icy dayside surface. Not having a detectable atmosphere, Charon almost certainly has such a “Moon-like” interaction, remaining primarily in the solar wind if Pluto’s interaction is weak but becoming totally engulfed if Pluto’s interaction is strongly “comet-like” and extends beyond Charon’s orbit at 17 RP (Pluto radii, ~1150 km). For a review of early studies of the solar wind interaction with Pluto see Bagenal et al. (1997), which also reviews the implications of the unlikely possibility of Pluto having an intrinsic magnetic field. The solar wind is supersonic so that when the flow impinges on an obstacle (such as the magnetosphere of the Earth or other planet) an upstream bow shock must form to slow and deflect the supersonic (actually superfast-mode magnetosonic) plasma. The weak interplanetary magnetic field (IMF) at 30 AU (see Table I for typical solar wind properties near Pluto) and heavy ions formed by photoionizing the heavy molecules of Pluto’s escaping atmosphere have very large gyroradi (~500 RP). The net results of these non-fluid or kinetic effects are to make the bow shock a thick transition region and to make the shape of the interaction region asymmetric where the direction of asymmetry is governed by the direction of the IMF. Recent simulations of the solar wind interaction with a strongly escaping 3 atmosphere have necessarily been 3D and have either taken a multi-fluid approach (solar wind proton fluid and pick-up ion fluid) or a hybrid approach (electron fluid, ion particles) (Harnett et al. 2005, Delamere and Bagenal, 2004). Table I. Typical Solar wind Interaction Properties at Pluto (at 30 AU), taken from Bagenal et al. (1997) using Rp=1150 km. Magnetic field 0.2 nT Proton density 0.01 cm-3 Solar wind speed 450 km/s Proton temperature 1.3 eV Alfvén Mach number ~45 Sonic Mach number ~40 Proton gyroradius 23,000 km (~20 Rp) N2+ gyroradius 658,000 km (~550 Rp) Ion inertial length 2280 (~2 Rp) Electron inertial length 53 km Figure 1. Solar wind interaction with Pluto for (a) low atmospheric escape rate and (b) high atmospheric escape rate. The dots indicate ions produced by the photoionization of Pluto’s escaping atmosphere. These pick-up ions will move perpendicular to both the magnetic field (B) and solar wind (upwards in the top two diagrams). Note that while the IMF tends to be close to 4 tangential to Sun direction, the sign of the direction varies on timescales of days. The asymmetries of the interaction will flip as the magnetic field changes direction. Below we discuss how the current understanding of Pluto’s atmosphere leads us to expect a more comet-like interaction at the time of the New Horizons flyby in 2015. Through measurements of bulk properties of the solar wind (flow, density, temperature) as well as the energy distribution of solar wind and pick-up ions, the SWAP instrument will not only characterize the solar wind interaction with Pluto but will also allow us to determine the global rate of atmospheric escape. In the comet-like scenario, variations on the scale of the interaction region can be substantial over periods of days, and a factor of ~10 variations in the solar wind flux can change the size of the interaction region from a few to more than 20 RP. It is therefore critical to measure the solar wind for several solar rotations (~26 days per rotation) before and after the flyby in order to characterize the most likely external solar wind properties during the actual encounter period. Furthermore, since the strong asymmetry of the interaction depends on the direction of the IMF, our analysis of SWAP data will need assistance from increasingly capable models of solar wind structure based on plasma and magnetometer data from spacecraft elsewhere in the solar system. 2.1 Atmospheric Escape The exact nature of Pluto’s plasma interaction is critically dependent on the hydrodynamic escape rate of the atmosphere from its weak gravity. Escaping neutrals are photoionized by solar UV (or, less likely, suffer an ionizing collision). Freshly ionized particles experience an electric field due to their motion relative to the IMF (that is carried away from the Sun at ~400 km/s) and are accelerated by this motional electric field, extracting momentum from the solar wind flow. This electrodynamic interaction modifies the solar wind flow. Estimates of Pluto’s atmospheric escape rate, Q, vary substantially: McNutt (1989) estimated 2.3-5.5 x 27 -1 10 s for CH4-dominated outflow, Krasnopolsky (1999) found a hydrodynamic 27 -1 outflow of N2 of 2.0-2.6 x 10 s , while Tian and Toon (2005) derived values for 28 -1 N2 escape as high as 2 x 10 s .